Technical Field
The invention relates generally to the art of heavy-duty vehicles. More particularly, the invention is directed to a steerable axle/suspension system for a heavy-duty vehicle. More specifically, the invention is directed to a steering centering/damping mechanism for a steerable axle/suspension system for a heavy-duty vehicle, which includes a mechanical steering centering/damping mechanism that provides a positive steering centering force at a zero steer angle to reduce wheel wobble at zero steer angle. The mechanical steering centering/damping mechanism also provides a steering centering force that increases with increasing steer angle, but with less force intensity at higher steer angles than prior art mechanical steering centering/damping mechanisms to decrease wheel slippage or tire scrubbing and increase tracking efficiency of the steerable axle suspension/system during vehicle turning. The combination of decreased wheel wobble and decreased tire scrubbing increases tire life and decreases vehicle maintenance costs. In addition, the steering centering/damping mechanism of the present invention includes reduced complexity, eliminates wear items and/or parts that can potentially fail, and provides improved packaging to allow the system to be more easily adapted to heavy-duty truck applications.
Background Art
Heavy-duty vehicles such as tractor-trailers and straight trucks typically include multiple axle/suspension systems that are longitudinally spaced along the vehicle to create ride stability. Federal Bridge Law directs that in order to transport the maximum load allowed, additional auxiliary axles are required at specified longitudinal spacing. Auxiliary axle/suspension systems may be non-liftable or liftable and steerable or non-steerable, depending on the specific application. Steerable axle/suspensions systems are well known in the art. It is also well known in the art that steerable axle/suspension systems are often capable of being lifted. Hence, for the purpose of simplicity, reference herein will be made to steerable axle/suspension systems with the understanding that steerable axle/suspension systems may optionally include a lift assembly.
The actual lifting of the lift axle is performed by the transversely-spaced suspension assemblies that are associated with the lift axle, with such steerable lift axle/suspension systems being well known in the art. The lift axle/suspension system typically is operated by a control system that is in turn actuated by a switch, commonly referred to as a lift switch, which is manually triggered by the operator of the vehicle. Lift switches traditionally have been installed in the cab of the vehicle for proximity to the operator. This prevents an operator from having to exit the cab, which can be particularly inconvenient in circumstances such as inclement weather.
In addition, the steerable axle/suspension system typically is located at about the center of the truck or trailer in the fore-aft direction and usually uses a single tire on each of the wheels at the axle ends. Such single tires each have a large area of contact with the ground. In a tight turning maneuver, the central positioning of the steerable axle/suspension system combines with the large area of contact of the tires and the severe angle between the tractor and the trailer, thereby causing the wheels of the steerable axle/suspension system to act as a pivot point.
When the vehicle is moving or operating in a reverse direction, the steerable axle/suspension system must be either lifted or locked into a straight position in order to allow for safe maneuvering of the vehicle. If the steerable axle/suspension system is lifted while operating in a reverse direction, then once the vehicle is again moved in a forward direction, the steerable axle/suspension system usually must be lowered back into contact with the ground surface. If the steerable axle/suspension system is locked into a straight position, or locked mode, while moving in a reverse direction, then once the vehicle is again moved or operated in a forward direction, the steerable axle/suspension system should be unlocked, or placed into a steering mode, so that it can once again steer as it is intended to do while moving in a forward direction.
Auxiliary steerable axle/suspension systems are generally self-steering. The wheels of such systems are forced to turn due to tracking forces imparted on the wheels by nature of the positioning of the steerable axle/suspension system relative to the tractor trailer during a vehicle turn, as well as each wheel's large area of contact with the road surface, and are not manually turned by a vehicle operator, as is known in the art. For purposes of the description contained herein, it is understood that the term “steerable axle/suspension system” will encompass “auxiliary self-steering axle/suspension systems.” Because auxiliary steerable axle/suspension systems are typically self-steering, during a vehicle turn the wheels of the axle/suspension system may experience improper tracking, referring to instances when the wheels of the axle/suspension system do not return to a position perpendicular to the vehicle axle, or a zero steer angle, by the time the vehicle has completed a turn. If the wheels do not return to a zero steer angle by the time the vehicle has completed a turn, the wheels of the steerable axle/suspension system can be dragged by the tractor trailer, generally referred to as tire scrubbing in the art, and as a result experience excessive wear on the tires of the steerable axle/suspension system.
To minimize tire scrubbing of the wheels of a steerable/axle suspension system due to improper tracking during a vehicle turn, prior art steerable axle/suspension systems often include steering centering mechanisms which aid in returning the wheels to a position perpendicular to the vehicle axle, or zero steer angle, by the time a vehicle completes a turn and is once again moving in a forward direction. One such prior art mechanism utilizes a pair of oil filled stabilizer shocks including coil over springs that are each connected at one end to a respective arm extending from each steering knuckle, and are connected at a second end to a mounting point on the axle body toward the central longitudinal axis of each respective side of the axle/suspension system. For example, as the vehicle encounters a left turn, the coil over stabilizer shock attached to the steering knuckle arm of the steering assembly on the inside of the vehicle turn radius is compressed because the arm attached to the steering knuckle turns inward relative to the axle body of the axle/suspension system to which the second end of the stabilizer is attached. Conversely, the coil over stabilizer shock attached to the steering knuckle arm of the steering assembly on the outside of the vehicle turning radius is extended because the arm attached to the steering knuckle turns outward relative to the axle body of the axle/suspension system to which the second end of the stabilizer is attached. Because of the extension and compression of the coil over spring, both the extended stabilizer and the compressed stabilizer exhibit a positive steering centering force which increases with increasing steer angle, with equilibrium of the system being achieved when the wheels of the steerable axle/suspension system are at a zero steer angle, as is known in the art. While the coil over springs provide steering centering to the steerable axle/suspension system, the oil and valving internal to the stabilizer shocks serve to reduce inputs that can lead to wobble or shimmy of the steerable axle/suspension system during operation of the vehicle.
Although coil over stabilizer shocks provide adequate steering centering/damping during a vehicle turn, when the steerable axle/suspension system is near a straight alignment or zero steer angle, the centering forces imparted by the coil over springs of the stabilizer shocks on the driver side and curb side steering assembly of the axle/suspension system are very low due to the spring design. More specifically, when the shocks are installed on the axle/suspension system, they are installed in a compressed state, with each imparting a near equal outboard force on a respective driver side and passenger side steering knuckle arm Because each of the shocks imparts a near equal outboard force on its respective steering knuckle arm, the forces essentially cancel each other, resulting in a net centering force of about zero at a zero steer angle. As a result, coil over stabilizer shocks may experience some amount of wheel wobble or shimmy at the wheels attached to the auxiliary axle/suspension system near the zero turn angle due to the about zero centering force. This can result in uneven tire wear, increased wear on the steerable axle/suspension system, and increased vehicle maintenance, as is known in the art. Additionally, coil over stabilizer shocks are considered wear items and must be replaced at regular intervals, resulting in increased maintenance costs. Coil over stabilizer shocks also generally exhibit steering centering forces with undesirably high intensities at increased steer angles, which can decrease tracking efficiency during turns and increase the potential of tire scrubbing of the wheels of the axle/suspension system. In addition, the location of the coil over stabilizer shocks can potentially have adverse effects on suspension packaging or spacing as they are relatively large and two coil over stabilizer shocks are required for each steerable axle/suspension system.
Other prior art steering centering/damping mechanisms for steerable axle/suspension systems utilize a pneumatically controlled mechanism to constantly apply a steering centering force to each of the vehicle steering assemblies, referred to as pneumatic steering centering/damping mechanisms, and for purposes of the description contained herein, are not considered mechanically operative in nature. Such pneumatic steering centering/damping mechanisms typically employ an air spring that connects to a pair of locking arms which interface with a bracket clamped to the tie rod. The pressure inside the air spring applies a steering centering force through the tie rod equally to each connected steering assembly. As the steer angle of the steerable axle/suspension system is increased, the air spring is compressed which in turn increases the centering force. Steering centering/damping reaction forces associated with this type of mechanism can be adjusted to fit the steerable axle/suspension system application by adjusting the mechanism's operating air pressure.
Such systems can provide a positive steering centering force at a zero steer angle, thus minimizing the inputs that can lead to wheel wobble while the vehicle is traveling in a straight direction. However, these pneumatic steering centering/damping mechanisms include known potential failure modes that can affect their operation, which include: loss of air pressure due to a variety of component failures, wear of the air spring, and introduction of water, ice or other contaminants into the system thereby affecting performance. Additionally, the packaging of the air spring and its associated bracketry is quite large and can potentially limit its use to only trailer applications, where space is more available. In truck applications, where the steering centering/damping mechanism must accommodate the vehicle drive shaft, such pneumatic steering centering/damping mechanisms generally cannot be used.
Thus a need exists in the art for a mechanically operated steering centering/damping mechanism that provides a positive steering centering force while the steerable axle/suspension system is at zero steer angle, as well as an increasing steering centering force as the steer angle of the vehicle steering assemblies increase, but with less force intensity than prior art mechanical steering centering/damping mechanisms, to reduce wheel wobble at zero steer angle and decrease tire scrubbing and increase tracking efficiency of the steerable axle/suspension system. The steering centering/damping mechanism of the present invention satisfies these needs by employing a steering centering/damping mechanism which includes a preloaded spring assembly integrated into each steering assembly which applies a constant positive centering force on each steering assembly at zero steer angle to resist the effects of inputs that lead to suspension wheel wobble and shimmy, and also offers increasing steering centering force on the steering assemblies as the steer angle increases with less force intensity, which decreases tire scrubbing and increases the tracking efficiency of the steerable axle/suspension system during a vehicle turn. The combination of decreased wheel wobble and decreased tire scrubbing increases tire life and decreases vehicle maintenance costs. The steering centering/damping mechanism of the present invention also eliminates wear items, such as coil over stabilizer shocks, and/or parts that can potentially fail, such as those of pneumatic steering centering/damping mechanisms. Additional benefits include the ability to utilize the steering centering/damping mechanism of the present invention in truck applications where the drive shaft of the vehicle limits packaging space. Moreover, the steering centering/damping mechanism of the present invention can potentially reduce the required packaging envelope fore to aft as coil over shock absorbers are no longer required. The steering centering/damping mechanism of the present invention also includes reduced complexity, which reduces weight and cost over the more complicated and bulky pneumatic steering centering/damping mechanisms of the prior art utilized in trailer applications.